(b) The gross longshore sediment transport rate is given by

1

* *Q *(*t*) * *dt*

(III-2-19)

where To is the length of record, often taken to be greater than 1 year, and |QR| is the absolute magnitude of

the longshore sediment transport rate. The gross longshore sediment transport rate is always defined positive.

When the gross longshore sediment transport rate is multiplied by the time period, the derived quantity

represents the total volume of sediment passing through a plane perpendicular to the shoreline regardless of

the direction. The net longshore sediment transport rate may be either positive or negative.

(c) Although net and gross longshore sediment transport rates are often the most meaningful quantities

for use in engineering design, the variability of the longshore sediment transport on much shorter time

intervals is often of critical concern. As an example, if a channel is to be maintained clear of sediment, the

pump(s) necessary to dredge the channel would need to be sufficiently large to handle the instantaneous

maximum rate of longshore rate of sediment transport (that is projected to reach the channel). Typically,

sizing for the maximum instantaneous rate of sediment transport would be economically unfeasible, and

therefore some type of temporary sediment storage would be provided to allow for reduced pump sizing.

Optimizing of the pump size and the provided sediment storage structures (possibly a groin, a breakwater,

or some combination of both) will require knowledge of the fluctuating longshore sediment transport

rates.Figure III-2-8 provides an example of the variability of annual rates of (net) longshore energy flux factor

(which is proportional to the longshore sediment transport rate as given in Equation 2-2) for three locations

on the East Coast of the United States as computed using 20 years of hindcast wave climate.

Longshore Sediment Transport

III-2-27

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